key: cord-0732718-f5tl62x9
authors: Magro, Cynthia; Nuovo, Gerard; Mulvey, Justin; Laurence, Jeffrey; Harp, Joanna; Neil Crowson, A.
title: The Skin as a critical window in unveiling the pathophysiologic principles of COVID-19
date: 2021-07-22
journal: Clin Dermatol
DOI: 10.1016/j.clindermatol.2021.07.001
sha: a0a7307ec2bc12410df7a5dd32037488232eb16b
doc_id: 732718
cord_uid: f5tl62x9

The severe acute respiratory distress syndrome-associated coronavirus-2 (SARS-CoV-2), the etiologic agent of Coronavirus disease 2019 (COVID-19), is a single-stranded RNA virus whose sequence is known. COVID-19 is associated with a heterogeneous clinical phenotype ranging from asymptomatic to fatal disease. It appears that access to nasopharyngeal respiratory epithelia expressing angiotensin-converting enzyme (ACE) 2, the receptor for SARS CoV-2, is followed by viral replication in the pulmonary alveolar septal capillary bed. We have shown in prior studies that incomplete viral particles, termed pseudovirions, dock to deep subcutaneous and other vascular beds potentially contributing to the prothrombotic state and systemic complement activation that characterizes severe and critical COVID-19. A variety of skin rashes have been described in the setting of SARS-CoV-2 infection and more recently, following COVID-19 vaccination. The vaccines deliver a laboratory synthesized mRNA that encodes a protein that is identical to the spike glycoprotein of SARS-COV-2 allowing the production of immunogenic spike glycoprotein that will then elicit T cell and B cell adaptive immune responses. In this paper we review an array of cutaneous manifestations of COVID-19 that provide an opportunity to study critical pathophysiologic mechanisms that underlie all clinical facets of COVID-19 ranging from asymptomatic/mild to severe and critical COVID-19. We classify cutaneous COVID-19 according to underlying pathophysiologic principles. In this regard we propose two main pathways: 1) complement mediated thrombotic vascular injury syndromes deploying the alternative and mannan binding lectin pathways in the setting of severe and critical COVID-19 and 2) the robust T cell and type I interferon driven inflammatory and humoral driven immune complex mediated vasculitic cutaneous reactions seen with mild and moderate COVID-19. Novel data on cutaneous vaccine reactions are presented that manifest a clinical and morphologic parallel with similar eruptions seen in patients suffering from mild and moderate COVID-19 and in most cases represent systemic eczematoid hypersensitivity reactions to a putative vaccine based antigen. Finally, we show for the first time the localization of human synthesized spike glycoprotein following the COVID-19 vaccine to the cutaneous and subcutaneous vasculature confirming the ability of SARS CoV-2 spike glycoprotein to bind endothelium in the absence of intact virus.

disease. It appears that access to nasopharyngeal respiratory epithelia expressing angiotensinconverting enzyme (ACE) 2, the receptor for SARS CoV-2, is followed by viral replication in the pulmonary alveolar septal capillary bed. We have shown in prior studies that incomplete viral particles, termed pseudovirions, dock to deep subcutaneous and other vascular beds potentially contributing to the prothrombotic state and systemic complement activation that characterizes severe and critical COVID-19. A variety of skin rashes have been described in the setting of SARS-CoV-2 infection and more recently, following COVID-19 vaccination. The vaccines deliver a laboratory synthesized mRNA that encodes a protein that is identical to the spike glycoprotein of SARS-COV-2 allowing the production of immunogenic spike glycoprotein that will then elicit T cell and B cell adaptive immune responses. In this paper we review an array of cutaneous manifestations of COVID-19 that provide an opportunity to study critical pathophysiologic mechanisms that underlie all clinical facets of COVID-19 ranging from asymptomatic/mild to severe and critical COVID-19. We classify cutaneous COVID-19 according to underlying pathophysiologic principles. In this regard we propose two main pathways: 1) complement mediated thrombotic vascular injury syndromes deploying the alternative and mannan binding lectin pathways in the setting of severe and critical COVID-19 and 2) the robust T cell and type I interferon driven inflammatory and humoral driven immune complex mediated vasculitic cutaneous reactions seen with mild and moderate COVID-19. Novel data on cutaneous vaccine reactions are presented that manifest a clinical and morphologic parallel with similar eruptions seen in patients suffering from mild and moderate COVID-19 and in most cases represent systemic eczematoid hypersensitivity reactions to a putative vaccine based antigen. Finally, we show for the first time the localization of human synthesized spike glycoprotein following the COVID-19 vaccine to the cutaneous and subcutaneous vasculature confirming the ability of SARS CoV-2 spike glycoprotein to bind endothelium in the absence of intact virus.

The severe acute respiratory distress syndrome-associated coronavirus-2 (SARS-CoV-2) is the etiologic agent of Coronavirus disease 2019 . It is a single-stranded RNA (ribonucleic acid) virus 1 and represents one of the seven beta coronaviruses 2 that causes human disease, although the natural reservoirs are the bat and the pangolin. 3 The SARS CoV-2 infection in humans is associated with a heterogeneous clinical phenotype 4 that ranges from asymptomatic cases to mild disease to a severe and potentially fatal illness. It is this unpredictability in an individual's response to the virus that has caused so many of us to adopt a new pattern of daily interactive existence until herd immunity is reached.

Organ failure, particularly a progressive, therapy-resistant acute respiratory distress syndrome (ARDS) 5 and acute renal insufficiency 6 along with a hypercoagulable 7 state portend the worst prognosis and qualify as critical COVID-19. SARS-CoV-2 is highly contagious, especially the more recently identified strains originating in England, Brazil and South Africa. 8 Its incubation period ranges from 2 to 14 days following spread via inhaled respiratory droplets or droplet nuclei, and less likely through fomite contamination. The name given to the virus that causes COVID-19, SARS-CoV-2, emphasizes the structural similarity to the original SARS coronavirus that was responsible for the 2003 to 2004 pandemic. The earlier contagion was more virulent and less transmissible while the new strain and other related mutant strains are more contagious but less virulent.

Common to the clinical presentation of SARS CoV and SARS CoV-2 are fever, dry cough, myalgias, headache, and diarrhea in most patients. In the setting of SARS CoV-2, the majority of patients have a relatively mild, self-limited illness. Roughly 20% of patients have more serious disease that can progress to respiratory compromise necessitating supplemental oxygen either via nasal cannula or ventilator support, fulfilling criteria for severe and critical COVID-19 respectively. 9 The pathophysiology that underlies the transition from moderate COVID-19 disease to critical and severe COVID-19 is complex, but a diminished type I interferon response in the earlier phase of the infection likely contributes to a more aggressive clinical course by permitting unchecked viral replication in infected cells. 10, 11, 12 The excessive microvascular replication of virus in the lung contributes significantly to septal capillary compromise but also results in extrapulmonary pseudovirion dissemination with further extrapulmonary vascular sequelae. 13 We had the opportunity to examine the skin, lung and various other organs systems including the brain, liver, kidney and heart in a number of patients who had mild, moderate, severe, critical and fatal COVID-19. A common theme that emerged is one of complement-mediated microvascular injury 13 and a generalized procoagulant state in the absence of intact virus within endothelium outside of the lung in patients with severe and critical COVID-19. We first recognized and identified a pattern of tissue injury in the skin and lung diagnostic of complement-mediated microvascular injury in patients with severe and critical COVID-19.

Subsequently we extended our studies to other organ systems including the heart, liver, kidney, brain and placenta. 14, 15 We discovered localization of spike glycoprotein and other viral capsid proteins to microvessels which expressed the receptor for spike glycoprotein, namely, angiotensin-converting enzyme 2 (ACE2). The greatest extent of ACE2 and viral capsid localization was in the septal capillaries of the lung, deeper dermal and subcutaneous capillaries and venules of both diseased and normal-appearing skin as well as the microvessels of the brain.

There was also evidence of complement pathway activation 14 in these ACE2 positive microvascular beds expressing viral capsid protein based on the localization of C3d, C4d, MASP-2 (mannan-binding lectin-associated serine protease-2), and C5b-9 within the microvasculature.

Spike glycoprotein exhibits glycan moieties that are able to interact with mannan-binding lectin (MBL) leading to MBL pathway activation. 16, 17 In particular, the SARS-CoV-2 S spike glycoprotein exhibits surface oligomannose-type N234 and N709 that are largely accessible to α-1,2-mannosidases. Of the 22 sites on the S protein, eight contain substantial populations of oligomannose-type glycans, hence rendering spike glycoprotein the perfect substrate for the activation of the MBL pathway. Our demonstration of microvascular endothelial cell localization of C4d and MASP-2 within the lung and skin procured from patients with critical and fatal COVID-19 provided confirmation of the role of MBL pathway activation and also prompted therapeutic interventions in critically ill COVID-19 patients with eculizumab 18. Others have also highlighted the role of MBL activation in the pathogenesis of severe/critical COVID-19 including its role in the thrombosis and coagulopathy in critically ill patients with COVID-19. 19 , 20 We further concluded that the hypercytokinemia was a direct sequela of elaboration of the various cytokines including tumor necrosis factor (TNF) alpha, MBL, caspase 3 and interleukin 6 (IL-6) from endothelium possibly due to MBL pathway activation. 14 19, 20 The addition of human MBL along with MBL-associated serine proteases complex or recombinant human MBL to cultures of human umbilical vein endothelial cells stimulated with bacterial lipopolysaccharide triggers enhanced secretion of select cytokines and chemokines including interleukin 8 (IL-8), IL-6 and monocyte chemoattractant protein-1 by endothelium indicative of an independent role of MBL activation in promoting an activated inflammatory phenotype in the endothelium. 21 The end result of this state of complement activation and hypercytokinemia is one of endothelial cell injury and microvascular thrombosis. 14 We postulated that the activation of the complement pathway and enhanced IL-6 expression on endothelium contributed to the generalized procoagulant state given the cross talk between the complement pathway and the coagulation pathway and the ability of endothelial based IL-6 to promote platelet aggregation. Other than the lung and nasopharynx, there is no evidence of active viral replication in the microvascular beds despite the localization of viral capsid protein to the endothelium. Hence, endocytosis of viral protein as a pseudovirion is nonetheless capable of activating endothelium through the complement pathway and inducing the expression of certain cytokines such as IL-6 likely through MBL activation. 14, 22 A variety of skin rashes have been described in the setting of SARS-CoV-2 infection and more recently, following the COVID-19 vaccine. Up to 20% of patients with active infection are said to manifest skin rashes. 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 Amongst the clinicopathological reaction patterns reported to date include a confluent erythematous maculopapular-morbilliform rash, 33 41 and vascular injury patterns including livedo reticularis, 42 thrombotic retiform purpura, 27, 43 acral purpuric nodules similar to idiopathic perniosis (chilblains) 43, 44, 45, 46, 47, 48, 49, 50 and a unique acroischemic lesion occurring in patients with moderate COVID-19 heralded by lesions that mimic perniosis clinically but exhibit paucicellular ischemic alterations in association with dilated blood vessels devoid of thrombotic and/or vasculitic change. 51 The main types of skin findings we encountered include those directly attributable to microvascular complement activation including thrombotic retiform purpura encountered in the severe and critical COVID-19 patient 13,27,43,52qw and the subclinical microvascular complement deposition 53 and subtle microvascular thrombosis observed in biopsies of normal deltoid skin in patients with severe and critical COVID-19. The latter findings in normal skin support the diagnosis of systemic complement pathway activation mirroring the microvascular complement pattern encountered in other catastrophic complement mediated vascular injury syndromes such as atypical hemolytic uremic syndrome 53, 54 . We have seen cutaneous signs of the generalized procoagulant state 56 operational in patients with significant COVID-19 represented by calciphylaxis and acute limb ischemia attributable to subcutaneous larger vessel arterial thrombosis 55, 56 . A hallmark of skin samples reflective of a procoagulant state and/or complement-mediated vascular injury was the lack of inflammation. In contrast, there are cases of cutaneous COVID-19 where significant T-cell enriched inflammation reflective of cellular immunity was observed; others have also reported on similar eruptions in various case reports and series. Among these cases were interferon driven COVID-19 perniosis 43, 44, 45, 46, 47, 48, 49, 50, 57 also referred to as "COVID toes," a robust lymphocyte inflammatory lesion resembling histiocytoid Sweet's syndrome, 58 PR, 59 and erythema multiforme. 37, 38, 39 We have also encountered leukocytoclastic vasculitis temporally associated with COVID-19 where a different limb of anti-viral immunity, namely a humoral immune response, likely was operative. 60 Calcinosis cutis reflective of calcium phosphate metabolism in the setting of COVID-19 associated renal insufficiency was identified in one patient. Reactions temporally associated with COVID-19 vaccine administration are also presented including Grover's disease, urticaria, urticarial vasculitis, systemic eczematoid hypersensitivity reactions, morbilliform interface reactions, perniosis and a patient with ulcerative colitis on mesalamine who developed a first onset of shingles one day after her first Pfizer vaccine dose. The clinical features, pathology and pathophysiology of the main cutaneous reactions encountered in our clinical practices will be discussed. A categorical approach to cutaneous COVID-19 based on underlying pathogenetic mechanisms that are likely key in their evolution is presented. The summaries of the pathophysiologies of Cutaneous COVID-19 are found in figures 22, 23, and 24.

Cornell Medicine and Regional Medical Laboratory in Tulsa Oklahoma are reviewed covering the time period since the start of the pandemic in March of 2020 to the time of the submission of this paper in June of 2021. The clinical histories and light microscopic findings were reviewed. As part of the routine work up in each case, additional immunohistochemical studies for the expression of myxovirus resistance protein, and C3d, C4d, and C5b-9 were evaluated in some cases. In addition, molecular studies including the viral capsid protein, ACE-2 assessment, cytokine expression, and the SARS Coronavirus RNA were conducted as supplementary research tests in certain cases. The methodology has been previously reported. 13, 14, 43 The study is covered under IRB protocol 20-02021524. MICROVASCULAR INJURY DUE TO SPIKE GLYCOPROTEIN ACTIVATION OF   THE COMPLEMENT PATHWAY IN THE SETTING OF A BLUNTED INTERFERON   RESPONSE (SEE TABLE 1) 

We encountered 9 patients with cutaneous thrombotic microvascular disease in the setting of severe and critical COVID-19 who had progressed from moderate to critical COVID-19; some of the cases have been previously reported 13, 27, 43 ; each had acute respiratory failure and became ventilator dependent with PCR (polymerase chain reaction) proven SARS CoV-2 infection.

Typically, after a few days on ventilator support the patients developed retiform purpura primarily at acral sites excluding 2 patients where the eruption was confined to the buttock and thigh areas respectively. The retiform rash was temporally associated with their worsening clinical features ( figure 1a, 1b) . The patients were adults ranging in age from 32 years of age to 73 years of age without a sex predilection. There were no known risk factors for the development of an acute thrombotic diathesis such as antiphospholipid antibodies, antibodies to heparin, warfarin administration and sepsis although some were diabetic. Four of the 9 patients died from organ failure related to COVID-19.

In all cases the biopsies showed a pauci-inflammatory thrombogenic vasculopathy affecting capillaries and venules, while in half of cases a thrombotic diathesis was observed in arterioles and small arteries, a finding associated with stroke and death. In contrast with the striking degree of vascular thrombosis and endothelial cell injury there was a dearth of inflammatory cells save for one case showing tissue neutrophilia (figure 2) that we attributed to secondary ischemic change. Degenerative endothelial cell changes with endothelial cell sloughing were more conspicuous in the deeper dermis and subcutaneous fat. Secondary ischemic necrosis of the eccrine coil was observed.

The vascular thrombosis was reflective of microvascular complement activation based on extensive deposition of complement components including C3d, C4d, and C5b-9 13, 43 . In addition, in certain cases there was very focal deposition of MASP-2, the activated form of MASP that triggers the remainder of the MBL complement cascade eventuating in the formation of C5b-9. However, of all the complement stains available on paraffin embedded tissue MASP-2 is the least sensitive and hence exhibited the overall lowest level of detection. Also, there can be significant background staining contributing to the difficulty in its interpretation. C5b-9 is the end point of complement pathway activation and its presence could signify activation of any of the three complement pathways. C4d also is a sensitive marker expressed at significant levels in the cutaneous and subcutaneous microvessels in patients with severe and critical COVID-19. The C4d stain is a challenge to interpret due to strong staining of elastic tissue in the dermis, but deposition was most prominent in the fat where there is little or no background elastic tissue positivity as is seen in the dermis. We know that C4d is formed with either classic complement pathway or MBL complement pathway activation. There is no reason to believe that classic complement pathway activation is the basis of the microvascular complement deposition especially since there is a compromised adaptive immune response in patients with severe and critical COVID-19. The prominent C4d and C5b-9 deposition with or without MASP2 in cutaneous microvessels in patients with severe and critical COVID-19 was held to be reflective of MBL and alternative pathway activation (figure 3a,3b). In particular the alternative pathway will be triggered from injured cellular remnants that are formed when other complement pathways are activated 13, 14, 43, 53 .

In all thrombotic retiform purpura cases we showed localization of the SARS CoV-2 capsid protein within the endothelium including spike glycoprotein, capsid membrane and capsid envelope but without evidence of intact virus as determined through ultrasensitive RNA in situ hybridization using the RNAscope assay and ultrastructural studies (figure 4a,b) 13, 14, 43, 53 . The positive staining vessels were most apparent in the deeper dermis, adventitial dermis of the eccrine coil, and subcutaneous fat where a number of positive vessels was characteristically seen (figures 4a,b).The basis of the spike glycoprotein cell engagement with endothelium reflects the expression of the critical SARS CoV-2 receptor namely ACE2 by the endothelium of microvessels where it is most apparent in the deeper dermal and subcutaneous capillaries and venules (figure 4c). The viral capsid proteins binds to the endothelium of cutaneous vessels as a pseudovirion and not as an intact virus. The evidence for the latter point, the absence of intact virus, is based on the negative ultrasensitive RNA in situ hybridization studies for SARS CoV-2, the lack of staining for nucleocapsid and the absence of SARS CoV-2 virions ultrastructurally. It is important to understand that the cutaneous endotheliopathy that underlies the ischemic dermopathy syndromes related to severe and critical COVID-19 is not reflective of a true infectious endothelialitis.

The

We hypothesize that the basis of the microvascular complement deposition is due in part to the engagement of spike glycoprotein with MBL based on extensive C4d and C5b-9 deposition, both representing components of MBL activation. One could argue that this complement expression pattern pattern could also indicate classic complement pathway activation attributable to antibodies targeting the spike glycoprotein, but in our experience the typical granular deposition pattern for IgG (immunoglobin G) in microvessels that would corroborate this hypothesis has not been observed, 13, 14, 53 .

Furthermore at least in a few cases there was focal microvascular staining for MASP-2, a sign of MBL pathway activation. 13 As already alluded to, the adaptive immune response in these patients is impaired due to the blunted type I interferon signaling possibly related to interferon signaling so classic complement pathway activation would be unlikely. 61 Other authors studying the role of complement pathway activation in the pathogenesis of severe/critical COVID-19 have identified a subset of patients with strongly elevated MBL plasma levels correlated with thromboembolism and with high plasma D-dimer levels. 62, 63 However alternative complement pathway activation is also contributory to vascular injury and in fact has been hypothesized to be the main pathway of complement pathway activation by some authors. They have suggested that the spike glycoprotein converts nonactivator surfaces into activator surfaces by preventing the inactivation of cellsurface alternative pathway of complement (APC) convertase. 64 It has been suggested that the SARS-CoV-2 spike protein (subunit 1 and 2), but not the nucleocapsid protein, directly activates the APC. The authors discovered that complement factor Bb, a serine protease that forms with alternative pathway activation to bind to C3b to form the alternative pathway convertase, is increased in the supernatant from spike protein-treated cells. C5 inhibition prevents the accumulation of C5b-9, but not C3c, on cells.

Addition of factor H, a complement regulator protein that controls the alternative pathway, mitigates the complement attack. 64 The Mechanisms Underlying MBL and Alternative Pathway Activation.

One could further hypothesize that the release of cellular debris from MBL pathway activation is an independent activator of the alternative pathway. The mannose residues on the surface of a pathogen recognize certain sites on MBL which will result in MASP-2 activation and trigger the remainder of the pathway to eventuate in the formation of C5b-9 with subsequent endothelial cell injury and defining a potential pathophysiologic construct to explain thrombotic complications attributable to MBL activation. 65, 66 MASP-2 staining as a sign of MBL pathway activation was seen in skin biopsies in some cases of thrombotic retiform purpura 13 , but in general this particular stain has relatively low sensitivity compared to other complement stains such as C5b-9. SARS CoV-2 associated Spike glycoprotein is established to have specific sugar residues that are able to interact with MBL leading to MBL pathway activation 67 although as already discussed others have suggested alternative and classic complement pathway activation as the basis of COVID-19-associated complement pathway activation. Regardless of the varied and potential mechanisms of complement pathway activation, C5b-9 mediated microvascular injury is the common end point of vascular injury. Components of complement pathway activation serve as a trigger to coagulation pathway contributing to the microvascular thrombosis. In addition, if one is to assume that MBL pathway activation is an important pathway of complement activation there is synergistic activation of the alternative pathway. Hence, a significant component of the microvascular thrombosis in the skin and other organ systems could reflect systemic complement activation in concert with a generalized procoagulant state that underlies severe, critical and fatal COVID-19. Reflective of systemic complement activation is the presence of microvascular C5b-9 in apparently normal skin of the deltoid as will be discussed presently 13, 53 .

Thrombotic retiform purpura does not occur in the setting of mild or moderate COVID-19 but rather is a cutaneous manifestation of severe and critical COVID-19. We propose that this distinctive eruption is a sequela of the excessive viral replication in the lung leading to the release of pseudovirions into the circulation as a fuel to complement pathway and coagulation pathway activation. Also, a blunted type I interferon response revealed by the lack of myxovirus resistance protein staining in tissue sections in these patients is likely a key pathogenetic event leading to excessive viral replication in the lung 43 .

A characteristic feature of critical and severe COVID-19 is hypercytokinemia; among the elevated cytokines are IL-6, TNF alpha, caspase 3, and interleukin beta . 68 Given the lack of inflammation encountered in biopsies of thrombotic retiform purpura and in the various organs studied in patients who die from COVID-19, it would seem unlikely that the hypercytokinemia is derived from T cells, B cells or monocytes. We were able to show colocalization of endothelial based viral pseudovirions including spike glycoprotein (figure 4d) with IL6 (figure 4e,4f) and TNF alpha (figure 4g,4h) in biopsies of COVID-19 associated thrombotic retiform purpura. We discovered high levels of IL-6 expression in the endothelium in cutaneous biopsies procured from lesions of thrombotic retiform purpura. A similar pattern of IL-6 upregulation was observed in microvessels of normal deltoid skin from patients with severe, critical and fatal COVID- 19 . It has been demonstrated that IL-6 elaboration from endothelium can occur due to MBL activation. 69 IL-6 elaboration from endothelium has a procoagulant effect; the mechanisms are complex and beyond the scope of this paper; among potential IL-6 effects are platelet aggregation and activation. 70, 71 We are starting to examine furin expression in endothelium. Furin is the enzyme expressed by endothelium that cleaves the S1 and S2 subunits of spike glycoprotein and can facilitate the entry of virus into a cell. It is likely modulated by various local microenvironmental factors including anoxia but a genetic polymorphism in furin expression has also been postulated with higher levels leading to more severe disease because of its permissive effects on viral entry 72 . We have examined two cases of thrombotic retiform purpura and have shown high levels of furin expression in endothelium in contradistinction to three biopsies of normal skin in healthy adults with minimal furin expression.

Further studies examining furin expression in cases of severe and critical COVID-19 compared to other catastrophic complement syndromes such as atypical hemolytic uremic syndrome and normal skin samples are being conducted at the time of the completion of this paper (figures 4i and 4j).

We have examined normal deltoid skin samples in patients who were significantly symptomatic with COVID-19, most of whom had severe, critical COVID-19 and/or succumbed to COVID-19. In a number of the cases there was subtle microscopic evidence of endothelial cell injury characterized by degenerative endothelial cell changes and focal vascular thrombosis. (figure 5a) 53 . We discovered evidence of systemic complement pathway activation in all patients except two. In particular, within the deeper dermal and subcutaneous microvessels of apparently normal skin, there were significant deposits of C4d or C5b-9 compatible with systemic complement pathway activation (figure 5b).

In addition, there was colocalization of viral capsid protein including spike glycoprotein, membrane and envelope protein in a subset of these complement positive vessels emphasizing the role of viral capsid protein endothelial cell endocytosis as a key factor in microvascular complement pathway activation (figure 5c) 13, 14, 43, 53 . Many positive staining vessels were observed similar to what was observed in the biopsies procured from patients with thrombotic retiform purpura of severe and critical COVID-19 although quantitatively somewhat less. It was however much greater than the minimal microvascular localization of spike seen in skin biopsies of vaccinated patients and in the T cell rich cutaneous reactions seen in patients with mild COVID-19 best exemplified by COVID-19 associated perniosis 43 .

The endothelial cells in the microvessels of the normal skin samples expressed various cytokines that are known to be elevated in COVID-19 including TNF alpha and IL-6 emphasizing the important finding that the source of the hypercytokinemia is not one of T cell or monocyte derivation but is derived from activated endothelial cells possibly through the effects of MBL pathway activation (figure 5d) 14 . We hypothesize that the basis of viral capsid localization to the deep dermal and subcutaneous vessels reflects higher levels of ACE2 expression in these vessels and have illustrated ACE2, complement and cytokine localization to microvessels of the deeper dermis and subcutaneous fat. Docking of viral proteins in normal skin and fat could serve as a fuel for both alternative complement pathway activation and coagulation pathway activation given the known synergy and cross talk between the complement and coagulation pathways 14, 53 .

A procoagulant state contributes significantly to the morbidity and mortality of severe and critical COVID-19. Among the distinctive clinical features are excessively high d-dimer levels reflective of intravascular fibrin deposition and ischemic complications related to medium and larger vessel thrombosis exemplified by lower limb ischemia due to thrombotic arterial occlusion. 73, 74, 75 We encountered two diabetic patients who developed occlusive thrombotic arterial disease resulting in below-knee amputation, one of whom developed other thrombi in larger arteries including the abdominal aorta and its branches in the setting of anticardiolipin antibodies of all isotypes that resolved after she had recovered from COVID-19 at day 48 of her hospitalization (figure 6a).

Amputated lower extremity specimens showed extensive occlusion of small and medium-sized arterial and venous vessels and capillaries by fibrin thrombi (figure 6b,c). There was deposition of C3d, C4d, and The basis of the procoagulant state leading to this type of catastrophic complication in COVID-19 is multifactorial. We know that the complement pathway activation operational in significantly symptomatic COVID-19 patients can trigger the coagulation pathway through the established synergy between complement pathway activation and coagulation pathway activation. 13, 14, 43, 75 Secondly additional procoagulant factors such as cold agglutinins or anticardiolipin antibodies may emerge.

Finally, there is diminished effectiveness of CD59, the regulator protein for C5b-9, due to its glycosylation in the setting of diabetes mellitus leading to unchecked and overzealous C5b-9 deposition on endothelium which we know plays a significant role in a number of thrombotic microangiopathies . 76

Calciphylaxis is an ischemic dermopathy syndrome seen in select clinical settings including renal insufficiency, other conditions associated with calcium phosphate abnormalities and certain diseases such as inflammatory bowel disease, cirrhosis, and multiple myeloma. The cutaneous and subcutaneous ischemia is attributable to a subcutaneous obliterative calcific intimal arteriopathy and a thrombogenic microangiopathy targeting capillaries associated at times with calcific endothelial mummification; a procoagulant state is permissive to the bone forming microenvironment of calciphylaxis in part through activation of osteopontin by thrombin. 77 Not surprisingly we have encountered two cases of this obliterative calcific thrombotic angiopathy in the setting of significant COVID-19; pathogenetic synergies included the underlying procoagulant state and general state of endothelial cell dysfunction and the potential for renal insufficiency with its associated abnormalities in calcium phosphate metabolism. One of these cases has been previously reported 28 .

The first patient was a 64 year old man with end stage kidney disease. The patient had developed COVID-29 in early March of 2021; he was significantly symptomatic requiring supplemental oxygenation.

In this third week of March the patient developed progressive retiform plaques with lesions on the right fingers, back of the bilateral lower extremities, left ankle, and tip of the penis which increased in size since his hospitalization for COVID-19 ( figure 7a) . The biopsy showed a subcutaneous pauciinflammatory thrombogenic vasculopathy accompanied by endothelial and mural calcification in microvessels. Many cutaneous and subcutaneous vessels had docked spike glycoprotein with a parallel staining pattern observed for IL-6, caspase 3 and TNF alpha with the greatest extent of immunoreactivity observed for IL-6 and caspase 3. There was strong expression of ACE2 in dermal and subcutaneous microvessels mirroring the distribution of spike glycoprotein and cytokine microvascular expression.

The second patient, previously reported, was a 62-year-old woman with end-stage renal disease secondary to focal segmental glomerulosclerosis and diabetes mellitus who suffered 3 weeks of cough and worsening atraumatic tender bilateral lower leg pain accompanied by purpuric plaques and was found to be COVID-19 positive via SARS-CoV-2 RT-PCR. A skin biopsy demonstrated classic features of calciphylaxis (figure 7 b,c) . 28 Over and above the calcific thrombotic diathesis in vessels of the subcutaneous fat (figure 7b) there were overlying skin changes of microvascular and arterial thrombosis without calcification (figure 7d) reminiscent of thrombotic retiform purpura of severe and critical COVID-19. Immunohistochemical assessment for the SARS-CoV-2 envelope protein and ACE2 revealed significant positivity in a few vessels in the deeper dermis and subcutaneous fat (figure 7e). 28 In both cases there were extensive deposits of C5b-9 within the microvessels throughout the dermis and subcutaneous fat corroborating a state of systemic complement activation typical of severe and critical COVID-19. Renal insufficiency causing calcium phosphate abnormalities was thought causal of the metastatic calcification observed in this case . 78

SARS CoV-2 elicits a robust T and B cell response due to the immunogenicity of single-stranded RNA, the nucleocapsid and various capsid proteins such as envelope, membrane and spike glycoprotein. Certain distinctive cutaneous reactions in the skin reflect an interferon driven T cell response to the virus and are perhaps best and most commonly exemplified by COVID-associated perniosis/chilblains 43 

Kawasaki's disease like multiorgan inflammatory syndrome but a broad spectrum of cutaneous reactions could be indicative of a cellular immune response to a virus such as the common morbilliform exanthem . 79, 80 When the T cell driven immune response is intense, the virus is eradicated quickly from the nasopharynx and therefore PCR testing may be negative. An effective T cell response may obviate the need for a humoral adaptive response and therefore in this subset of patients with COVID-19, negative SARS COV-2 antibody results may be observed.

There have been numerous reports of patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection presenting with pernio-like (chilblains) lesions popularly called "COVID toes."

These are red or purple papules that are often tender or itchy and may blister or ulcerate (figure 8a,b) .

Primarily found in children, they may be the only manifestation of COVID-19 in these patients who are typically negative for SARS-CoV-2 PCR and antibody testing for reasons already outlined and exhibits a significant degree of overlap with familial chilblains. 43, 44, 45, 46, 47, 48, 49, 50, 80, 81, 82, 83 We have encountered a number of COVID-19 perniosis cases with histologic samples taken represented by both children and adults ranging from 15 to 65 years of age; some of these cases have been previously reported. There was no history of a similar eruption in these patients pre-COVID nor was there any history of Raynaud's phenomenon. Their serologic autoimmune studies were negative. 43 

Although COVID-19 associated perniosis has many features in common with idiopathic perniosis, a differentiating feature is the presence of necrotizing vasculitic changes in reticular dermal-based blood vessels. In idiopathic perniosis fibrin deposition is largely localized to dermal papillae capillaries and typically does not involve reticular dermal-based blood vessels. All cases of COVID-19 perniosis manifest a strong type I interferon signature revealed by intense human myxovirus resistance protein 1(MXA) staining similar to the classic interferonopathy of idiopathic perniosis (figure 8f). MXA is the surrogate type I interferon marker detectable in paraffin-embedded tissues. 43 Idiopathic/familial perniosis is associated with mutations in TREX1 and RNASEH2. 81, 84, 83, 85 Excessive interferon signaling is due to an accumulation of nucleases involved in removing nucleic acids that accumulate during apoptosis.

We studied the microanatomic distribution of viral protein expression in COVID-19-associated perniosis and found no expression of SARS CoV-2 protein in endothelium except one case where a single vessel exhibited docked spike glycoprotein as a pseudovirion. Only a few cells of probable monocytic lineage contained SARS CoV-2 protein and RNA.

The literature is conflicting regarding direct infection of endothelium by the virus in the setting of COVID-19-associated perniosis and the localization of viral capsid protein to endothelium. One group at Yale University published two studies in which they were unable to find nucleocapsid protein in endothelium which would corroborate our position that endothelial cell infection by viable SARS CoV-2 has not occurred 86 ; RT in situ PCR for spike glycoprotein and nucleocapsid protein were negative in all of their cases although three of the six cases had spike glycoprotein in endothelium. 87 They suggested that the spike glycoprotein was endocytosed as a pseudovirion but could result in endothelial cell injury through its interaction with ACE2 and eventuate in lesions of perniosis. Another study demonstrated a uniform pattern of endothelial expression of SARS CoV-2 in vessels in the superficial dermis which defines a pattern we have never seen including in cases of severe and critical COVID-19; in our experience the pattern of microvascular staining for SARS CoV-2 is a focal interrupted one with dominant localization of immunoreactivity within deeper vessels and subcutaneous fat. This uniform superficial microvascular staining pattern raises consideration in regards to background staining. 88 In one study the authors claimed evidence of direct viral infection of endothelium in lesions of chilblains by observing intracellular structures reminiscent of the SARS CoV-2 virion 88 ; in one letter to the editor, the authors indicated that their illustrations were more likely of clathrin coated vesicles, a normal intracellular structural organelle and not viral proteins. 89

COVID-19-associated perniosis-like lesions can be considered a form of secondary perniosis along with chilblain lupus of Hutchinson and perniosis in the setting of antiphospholipid antibody and cryopathy syndromes such as cold agglutinins and cryofibrinogens. 90 The negative PCR nasopharyngeal studies in most cases of COVID-19 perniosis may reflect the efficacy of an interferon driven monocyte and T cell response to clear the virus but which could also have deleterious consequences if unleashed in a multiorgan inflammatory context best exemplified by the multiorgan inflammatory syndrome resembling Kawasaki's disease seen in children infected with SARS CoV-2.

Sweet's syndrome is a reactive T cell and monocyte-driven dermatosis observed in diverse clinical settings including hematologic dyscrasias, autoimmune disease, various infections, inflammatory bowel disease and drug hypersensitivity. Microbial antigenic triggers include streptococcal sp., Staphylococcus aureus, yersinia sp., upper respiratory viral pathogens and vaccinations that provoke a mononuclear cell-driven cytokine milieu associated with a secondary influx of neutrophils into the skin. The hallmarks clinically are infiltrative violaceous plaques accompanied by fever. Extracutaneous Sweet's syndrome has been linked to underlying hematologic malignancy. 91, 92 We encountered a 51-year-old previously healthy man who presented with a papulovesicular rash for 5 days in the setting of a recent diagnosis of COVID -19 (figure 9a,b) . The light microscopy showed classic features of Sweet's syndrome, namely significant papillary dermal edema associated with interstitial neutrophilic and perivascular mononuclear cell infiltrates (figure 9 c,d,e,f); the mononuclear cells exhibited a monocyte-derived dendritic cell phenotype as revealed by immunoreactivity for CD4, CD11c, CD14, and myeloperoxidase.

Leukocytoclasia was noted. In areas the dominant infiltrate was a histiocytic one reflecting overlapping features of conventional Sweet's syndrome with histiocytoid Sweet's syndrome.

Upregulation in type I interferon signaling as revealed by MXA expression in endothelium, inflammatory cells and epithelial structures was observed (figure 9g). There were rare vessels positive for SARS CoV-2 spike glycoprotein (figure 9h). Another reported case was that of a 61-year-old woman who developed Sweet's syndrome concurrent with moderately severe COVID-19. She had significant pulmonary disease radiographically along with the skin eruption, fever, and arthralgias 58 . We surmise in these cases of COVID-19 associated Sweet's syndrome that the virally driven type I interferon response preferentially recruits Th1 cells with a resultant cytokine microenvironment conducive to the influx of neutrophils and monocytes.

The most commonly reported skin findings are similar to those encountered in typical viral exanthems such as maculopapular and morbilliform eruption and papulovesicular eruptions 93 and are likely pathogenetically similar being reflective of a Gell and Coomb's type IV immune reaction. The presentations clinically included eczematous dermatitis (three patients), pityriasis rosea (PR) (two patients), acute generalized exanthematous pustulosis (AGEP) (one patient) ( figure 10 a,b,c) and a pruritic papular eruption mimicking papular urticaria (two patients). The most common pattern was a subacute eczematous one invariably accompanied by a subtle cell-poor interface dermatitis consistent with a systemic eczematoid hypersensitivity reaction identified in four patients including two patients thought to have PR, AGEP (one patient), a low grade lymphocytic vasculitis (one patient), and papular urticaria (one patient) ( figure 11a,b) . The eruptions were temporally associated with the diagnosis of acute COVID-19 with most developing the cutaneous eruption within two weeks of diagnosis. The eruptions were self-limited and lasted from 2 days to 5 months. A similar spectrum of T cell driven reactions have been described by others 36, 37, 38, 39, 40 . In all cases, the patients were either asymptomatic or had mild COVID-19. In 6 of the cases we conducted spike glycoprotein, IL-6 and TNF alpha immunohistochemical stains. The results, similar to those seen with perniosis, showed only a few cells in the deeper dermis representing either endothelial cells or inflammatory cells expressing spike glycoprotein, IL-6 and TNF alpha.

One study in Spain examined over 300 patients with COVID-19 via a questionnaire submitted to various clinicians who reported a variety of maculopapular rashes including morbilliform eruptions and PR-like reactions along with erythema multiforme. Many of the patients had received drug therapy and therefore a drug reaction was possible. This paper did not address histologic findings; it would appear that in the majority of the cases the patients did not undergo a biopsy. 94

A humoral reaction to SARS CoV-2 may be the basis of neutrophilic urticarial responses including urticarial vasculitis and leukocytoclastic vasculitis observed during the course of acute COVID-19. In our experience, the patients developing this cutaneous complication have skin limited vasculitis with COVID-19 disease of mild to moderate severity. We surmise that the etiologic basis is similar to other forms of cutaneous leukocytoclastic vasculitis reflective of an Arthus type III immune complex reaction. In the setting of SARS CoV-2 the putative immune complexes likely comprise antibody complexed to capsid proteins and other components of nonreplicative virus released into the circulation. The circulating immune complexes would serve as a trigger for classic complement pathway activation leading to the release of neutrophil chemoattractants and vasopermeability factors with resultant vascular injury, inflammation, red cell extravasation and vasocentric fibrin deposition.

We encountered four cases of leukocytoclastic vasculitis (LCV) where there was a temporal association between antecedent COVID-19 and where other LCV triggers could not be uncovered. These included one case of bullous LCV, one case of LCV associated with trace cryoglobulins, one case of urticarial vasculitis, and one case of a mixed lymphocytic vasculitis and LCV. There was an additional case where the patient had been recovering from critical COVID-19 and where pseudomonas triggered IgA vasculitis developed. There are anecdotal case reports of cutaneous LCV in the setting of COVID-19.

A 49-year-old Dominican woman with a PCR-positive nasopharyngeal swab developed palpable purpura several weeks later at which time she had positive COVID-19-associated antibodies. She also had evidence of trace cryoglobulinemia. A second case was a 72-year-old man with moderate COVID-19 who developed palpable purpura two weeks after his initial diagnosis. Both had ground glass pulmonary opacities, but neither was hypoxemic, neither required supplemental oxygen ( figure 12 a,b,c) and therefore fulfilled criteria for moderate COVID-19. Two additional COVID-19 positive patients developed LCV after two weeks of mild COVID symptoms.

Skin biopsies demonstrated typical LCV changes in all four patients characterized by angiocentric neutrophilic infiltrates with leukocytoclasia and mural fibrin deposition ( figure 12  a,b, figure 13a,b) . One patient had classic IgA vasculitis triggered by pseudomonas infection in the setting of severe/critical COVID-19. One of the four cases had a significant degree of intervascular neutrophilia compatible with urticarial vasculitis. In another case there was a conspicuous vasocentric lymphocytic component therefore defining a hybrid leukocytoclastic and lymphocytic vasculitis albeit the dominant morphology was an LCV (figure 12 a,b,c) .

Marked C4d and C5b-9 deposition within microvessels suggested classic complement and/or MBL pathway activation. It is likely that the basis of the vasculitis in the setting of COVID-19 could be a combination of an Arthus type III immune complex reaction comprising viral-related protein bound to antibody that is generated as part of the adaptive immune response triggering the classic complement pathway. In one case virally triggered type III mixed cryoglobulinemia remains a possibility (case 1).

These cases showed spike glycoprotein and other viral capsid protein localization to microvessels and/or to extravascular mononuclear cells. The microvascular spike glycoprotein was minimal compared to the extent of spike glycoprotein localization seen in the setting of thrombotic retiform purpura of severe and critical COVID-19.

Only one case expressed viral RNA, largely localized to rare mononuclear cells.

In the case of IgA-associated vasculitis, we postulated that the trigger was pseudomonas septicemia. 

There are anecdotal reported cases of COVID-19-associated LCV in adults ranging from 29 to 64 years of age. The LCV developed one week to one month after the diagnosis of COVID-19. 

A number of vaccine candidates utilizing a reverse genetic system primarily targeting the spike glycoprotein of SARS-CoV-2 have been developed with the earliest allowable age for vaccination being weeks after vaccination. In nine cases the eruption developed within 1 week after receiving either the first or second dose including three cases where the eruption developed within 48 hours following the vaccine. In five cases the reaction was delayed, developing 9 days, 10 days, 3 weeks, 4 weeks, and 7 weeks after receiving the vaccine. Among the eruptions were localized erythema at the site of the vaccine (2 cases) eczematous dermatitis (six cases), morbilliform hypersensitivity (one case), urticaria (one case), lymphocytic vasculitis (one case), Grover's disease (one case), urticarial vasculitis (two cases), and perniosis (one case) ( figure 14a,b) (figure 15a) (figure 16 a) (figure 17 ) . In all cases the clinical impression was congruous with the histologic findings, and in all cases the eruptions resolved either spontaneously or with topical or systemic steroid therapy ( figure 14b) (15b,c,d) (figure 18) (figure   19a,b) . In cases resembling eczema the clue to the systemic nature of the hypersensitivity reaction was the concomitant interface dermatitis. Conversely in the morbilliform reaction the dominant interface pattern is accompanied by subtle eczematous changes. In addition, in one case there were pustules noted clinically although there was no histologic documentation of a pustular diathesis. There were no accompanying significant systemic symptoms. One patient had significant peripheral blood eosinophilia. 

The immunohistochemical stain to assess for spike glycoprotein was conducted in an attempt to document evidence of human myocyte synthesis and establish the ability of spike glycoprotein to dock to ACE-2 positive vessels as a pseudovirion, an hypothesis proferred as the basis of systemic complement activation in the setting of severe and critical COVID-19. We were able to document spike glycoprotein in the cutaneous microvasculature in all cases tested. In particular, there were occasional deep-seated vessels in the deep reticular dermis and fat that showed focal expression of spike glycoprotein in endothelium which is not surprising as it reflects the preferential expression of ACE2 in deeper dermal and subcutaneous microvessels. There were only one to rare positive staining vessels (figure 21).

A similar distribution was observed for IL-6, caspase 3, and or TNF alpha, but without significant microvascular complement deposition was observed ( figure 20) . The overall amount of spike glycoprotein in the microvessels was much less than what we observed in the setting of thrombotic retiform purpura of severe and critical COVID-19.

It is our impression that the hypersensitivity reactions associated with the vaccine are mild, self limited and are primarily type IV delayed dermal hypersensitivity reactions. Vasculitic cases however represent a humorally mediated reaction. If one makes the assumption that the vehicle of delivery is largely inert the newly synthesized foreign antigen namely the spike glycoprotein which is capable of eliciting a robust T cell and antibody driven response could be the trigger to these cases of post vaccine associated cutaneous hypersensitivity reflecting either a type IV immune response to small amounts of spike glycoprotein localized to cutaneous microvessels or an Arthus type III immune complex reaction comprising spike glycoprotein bound to antibody. The fact that certain cases exhibit a morphology that resembles the cutaneous manifestations of mild COVID-19 such as perniosis and PR perhaps provides The effectual interferon response will result in viral clearance and the relative absence of pseudovirions but can potentially elicit various forms of cutaneous type IV hypersensitivity.

We have long recognized the skin as a critical window for understanding disease. Being our most superficial organ, the skin provides our easiest access for studying COVID-19 and providing an unparalleled opportunity for unraveling key pathophysiologic principles of COVID-19. Patients had progressed from moderate COVID-19 to critical COVID-19; each had acute respiratory failure and were ventilator dependent. They had PCR proven SARS CoV-2 infection. Typically after a few days on ventilator support the patients developed retiform purpura that was invariably associated with their worsening clinical features(figure 1)

In all cases of COVID-19 associated thrombotic retiform purpura the biopsies show a relatively pauciinflammatory thrombogenic vasculopathy affecting capillaries and venules and in half of cases a thrombotic diathesis is observed in arterioles and small arteries, a finding that is associated with stroke and death. Illustrated is a small artery exhibiting an occlusive thrombus. In most cases at least in some of the sampled vessels there is accompanying endothelial cell injury and variable mural fibrin deposition. The thrombotic diathesis reflects complement mediated endothelial cell injury in concert with a generalized procoagulant state due to concomitant activation of the coagulation pathway reflecting the critical interplay between the complement pathway and coagulation pathway. The biopsy illustrated in 2b is from a patient whose clinical course is complicated by a stroke over and above respiratory failure. (H and E, 400x) . In 2a and 2c the patient was a 22 year old woman with morbid obesity who developed critical COVID-19(clinical lesions are illustrated in 1b). The lower power image captured in 2a(H and E, 100x) shows a relatively pauci-inflammatory thrombogenic vasculopathy associated with livedoid superficial vascular ectasia; the combination of the occlusive thrombotic diathesis and resultant vascular ectasia leads to the distinctive clinical lesion of thrombotic retiform purpura. In 2c the thrombotic diathesis was associated with signs of endothelial cell injury as revealed by proplastic endothelial cell alterations. As well a moderately dense vasocentric neutrophilic infiltrate was noted although without significant mural fibrin deposition. The neutrophilic infiltrates likely reflect the sequelae of the proinflammatory effects of C5a, a component of complement pathway activation.

The vascular thrombosis is reflective of microvascular complement activation based on the extensive degree of deposition of components of complement activation including C3d, C4d, and C5b-9. C4d is also a very sensitive marker that is expressed at significant levels in the cutaneous and subcutaneous microvessels in patients with severe and critical COVID-19. C4d is formed with either classic or mannan binding lectin pathway activation and is illustrated in a (figure 3a) (diaminobenzidine(DAB), 1000x). As already alluded to, both C4d and C5b-9 are present at significant levels within the microvessels of skin biopsies in patients with severe and critical COVID-19, a finding well exemplified by this case. C5b-9 is the effector mechanism of microvascular endothelial cell injury and is illustrated in b (figure 3b) (DAB, 400x).

In each of thrombotic retiform purpura cases, localization of the SARS CoV-2 capsid protein within the endothelium including spike glycoprotein, capsid membrane and capsid envelope is observed but not intact virus. Illustrated is spike glycoprotein endothelial cell localization within the deep dermal and subcutaneous microvessels in a patient who succumbed to COVID-19(a,b)(4a, red chromogen, 1000x)(4b, red chromogen 400x). At autopsy he had classic thrombotic retiform purpura which was biopsied. In this case and other similar cases the viral capsid protein localization is most apparent in the deeper dermal and subcutaneous microvessels and mirrors the distribution of ACE-2 expression in microvessels of the skin and subcutaneous fat(c)(DAB, 1000x). The deep dermal and subcutaneous vessels that expressed spike glycoprotein (4d) (DAB, 1000x) also show IL6 (figure 4e) (red chromogen, 1000x) and TNF alpha expression(4g) (red chromogen, 1000x) whereby endothelial cell colocalization of spike glycoprotein and IL6 and spike glycoprotein and TNF alpha can be demonstrated using a nuance software which converts the red chromogen stain into a red signal (4e)(red chromogen, 1000x) and the DAB stain into a green signal(4d); areas of colocalization appear yellow(4f and h). In another case of thrombotic retiform purpura there was very striking expression of furin within endothelium(4i) in contrast with its minimal expression in endothelium of biopsied normal deltoid skin in a healthy adult woman(4j).

In a number of the cases of apparently normal skin in patients with severe critical COVID-19 or fatal COVID-19 there is subtle microscopic evidence of endothelial cell injury characterized by degenerative endothelial cell changes and focal vascular thrombosis ( figure 5a)(H&E, 400x) .

Within the deeper dermal and subcutaneous microvessels of apparently normal skin procured from the deltoid area, there are significant deposits of C4d or C5b-9. illustrated are prominent microvascular deposits of C5b-9(figure 5b)(DAB, 400x). There is localization of viral capsid protein within the deeper and subcutaneous cutaneous microvasculature including spike glycoprotein, membrane and envelope although without any evidence of viral replication as revealed by the lack of any SARS CoV-2 RNA localization to endothelium. Illustrated is viral capsid membrane localization to endothelium(figure 5c) (DAB, 1000x). In addition, the endothelial cells express various cytokines that are known to be elevated in COVID-19 including TNF alpha and IL-6. In this image there is striking localization of IL-6 to the endothelium of microvessels in a microanatomic distribution that mirrors complement and viral capsid protein expression(figure 5d) (DAB, 400x).

Clinical features:

Reflective of the striking procoagulant state observed in patients with severe and critical COVID-19 is the tendency toward multiorgan thrombosis including one characterized by arterial thrombotic occlusion of lower extremity vessels leading to lower limb ischemia. In the case illustrated the patient had multiorgan larger vessel thrombosis in the setting of COVID-19 complicated by antiphospholipid antibodies. The development of larger vessel thrombosis lead to lower extremity ischemia eventuating in the patient losing her leg below the knee. Illustrated is the explanted below the knee amputation specimen demonstrating ischemic gangrenous necrosis of the foot and distal leg(figure 6a). 

Both amputated lower extremity specimens show extensive occlusion capillaries, venules, small arteries and subcutaneous larger arteries by fibrin thrombi. A component of endothelial cell injury and mural fibrin deposition is noted in capillaries and venules(6b)(H&E, 400x) while the subcutaneous large vessel arterial occlusion is unassociated with similar endothelial cell alterations and hence largely reflective of an underlying procoagulant state as opposed to a thrombotic diathesis triggered by endothelial cell injury ( figure 6c)(H&E, 200x) . In the same vein the vascular complement deposition is largely localized to the endothelium and in a subendothelial cell array within microvessels (figure 6d)(DAB, 200x) while significant complement deposition is not observed in the larger arteries within the subcutaneous fat ( figure 6e) (DAB, 200x) . SARS-CoV-2 viral envelope and membrane (figure 6f)(red chromogen, 400x) proteins co-localized with TNF alpha and Il-6 shows a similar pattern of vascular localization as that noted for complement(figure 6g)(DAB, 400x) being localized to endothelium of microvessels but not larger occluded arteries. RNA studies to detect intact virions are negative. Immunohistochemical assessment for the SARS-CoV-2 envelope protein and ACE2 reveals significant positivity in a few vessels in the deeper dermis and subcutaneous fat. Illustrated is SARS CoV-2 capsid envelope protein(7e)(DAB, 400x) .

There have been numerous reports of patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection presenting with pernio-like (chilblains) lesions that popularly have been called "COVID toes." These are red or purple papules that are often tender or itchy and may blister or ulcerate as illustrated in 8a(figure 8a Courtesy Dr. Scott Sanders, New City, New York) The patient was a 51 year old previously healthy man who presented with a papulovesicular rash accompanied by fever (b). The biopsy showed classic features of Sweet's syndrome, namely significant papillary dermal edema(figure 9f) associated with interstitial neutrophilic and perivascular mononuclear cell infiltrates(figure 9 c,d,e,f)(9c,H&E, 100x)(d,e,f, H&E 200x).

Higher power magnification reveals that the dominant infiltrate is a histiocytic one confirmed phenotypically. The overall morphology is most consistent with Histiocytoid Sweet's syndrome (c,d,e,f) Upregulation in type I interferon signaling as revealed by MXA expression in endothelium, inflammatory cells and epithelial structures is observed( figure 9g)(DAB, 200x) . Spike glycoprotein localized to rare microvessels is identified (red chromagen 400x)(figure 9h).

A 59 year old woman presented with a widespread erythematous macular rash and was subsequently found to be PCR positive for Cov-Sars-2 by nasopharyngeal swab (courtesy Dr S. Rougas, Oklahoma City OK)(figure 10a).

Small (1-3 mm diameter) pustules were superimposed on an erythematous base in the fashion of acute generalized exanthematous pustulosis (AGEP)(figure 10b)

A discrete subcorneal and intra-epidermal pustule is present and corroborates the clinical diagnosis of AGEP (figure 10c) (H&E, 100 X)

A 52 year old woman developed a widespread morbilliform rash after testing positive for Cov-Sars 2 by nasophayryngeal PCR swab (Courtesy Dr. S. Rougas, Oklahoma City OK).

There is a cell poor lymphocytic interface dermatitis with slight epidermal spongiosis and patchy parakeratosis in the absence of tissue eosinophilia. The histology resembles a viral exanthem; a morbilliform drug eruption is the differential diagnosis (H&E, 400x).

The patient was a 72 year old man who had lethargy, fever and shortness of breath requiring supplemental oxygenation via nasal canula. The patient also developed palpable purpura. The patient was found to be SARS CoV-2 positive. A biopsy was performed of his skin rash which demonstrates a lymphocytic and neutrophil enriched necrotizing vasculitis. The biopsy shows a vasocentric lymphocytic and neutrophilic infiltrate that surrounds and permeates the vessel wall with evidence of vascular compromise characterized by mural fibrin deposition with attendant hemorrhage. (H&E, 100x) . The higher power magnification demonstrates an angiocentric and interstitial mixed infiltrate comprising lymphocytes, monocytes, and neutrophils. There is prominent endothelial cell swelling and as well there is prominent red cell extravasation corroborative of the diagnosis of a mixed leukocytoclastic and lymphocytic vasculitis. (H&E, 400x).

The patient was a 55 year woman who developed mild COVID. Approximately 10 days later the patient developed a purpuric bullous rash. There is massive papillary dermal edema with upper dermal hemorrhage visible from scanning magnification (H&E, 100X). The patient is a 38 year old woman who carries a diagnosis of lupus erythematosus. The patient has a history of limited discoid lupus erythematosus on the face that antedates her presentation by 12 years. The patient has been on plaquenil for a number of years. The patient had the COVID vaccine on 02/15/2021 and then the next day the patient developed redness and swelling in the suprapubic area of 02/16/21 and a more full blown rash on the back and abdomen about a week ago.

The biopsy shows a cell poor interface dermatitis. A low grade lymphocytic vasculitis was noted. In addition there was a supervening interstitial granulomatous component. A diagnosis was made of a post vaccine hypersensitivity reaction.(H&E, 400x)

The patient is a 34 year old woman with a pruritic rash that is generalized over the body. It developed after her second COVID Moderna vaccine on the 14 th of February. After the vaccine the patient developed body aches and chills for about two days. On the third day she developed a rash at the site of the vaccine and then it spread to the trunk, arms, and proximal thighs. The patient is healthy and does not take any medications. (H&E, 200x) 

The biopsy shows a hybrid interface and eczematous dermatitis along with a superficial lymphocytic purpuric vascular reaction. Scattered eosinophils were also noted. The overall morphology is characteristic for a systemic type IV hypersensitivity reaction. (H&E, 400x)

The biopsy exhibits spongiosis with lymphocytic exocytosis. A subtle interface dermatitis is identified. Focal parakeratosis with dyskeratosis is noted.

There is a lymphocytic purpuric vascular reaction accompanied by a few eosinophils. Overall, the biopsy is one that shows a spongiotic eczematous picture with very subtle interface dermatitis compatible with a systemic type IV hypersensitivity. (H&E, 400x) The biopsy demonstrates an inflammatory process within the dermis exhibiting urticarial-like features. In particular, there is a mixed infiltrate that is interstitial and perivascular composed of lymphocytes, monocytes, and neutrophils along with a few eosinophils with concomitant dermal edema. Inflammatory cells migrate through the vessel wall, although frank vasculitic changes are not seen. Nevertheless there is focal leukocytoclasia and evident N(17b, H&E, 200x) (17b, H&E, 400)

A 37 year old woman suffered first onset of shingles 1 day after her first dose of the Pfizer Covid 19 vaccine. No biopsy was taken as the clinical picture was considered pathognomic. The rash cleared after 7 days on Valtrex.

A 48 year old woman developed an urticarial tissue reaction days after receiving her first dose of the Moderna Covid 19 vaccine, and characterized histologically be dermal edema with an interstitial mixed inflammatory infiltrate comprising lymphocytes, histiocytes, and interstitial mast cells. There was demodex colonization of a hair follicle structure with lymphocytic exocytosis into the follicular epithelium (H&E, 200x). Case courtesy Dr. Miranda Smith, Tulsa OK 

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Maculopapular eruptions associated to COVID-19: A subanalysis of the COVID-Piel study

Exuberant bullous vasculitis associated with SARS-CoV-2 infection

Cutaneous leukocytoclastic vasculitis secondary to COVID-19 infection: A case report. Clin Case Rep

Leucocytoclastic vasculitis in a patient with COVID-19 with positive SARS-CoV-2 PCR in skin biopsy

Urticarial vasculitis in a COVID-19 recovered patient

Delayed Localized Hypersensitivity Reactions to the Moderna COVID-19 Vaccine: A Case Series

Cutaneous reactions reported after Moderna and Pfizer COVID-19 vaccination: A registry-based study of 414 cases

New-onset chilblains in close temporal association to mRNA-1273 vaccination. JAAD Case Rep

Cutaneous reactions reported after Moderna and Pfizer COVID-19 vaccination: A registry-based study of 414 cases